An opticians state that they cannot replace the lenses in a brand of prescription safety glasses for health and safety reasons. You have to buy a new frame from them.
Safety glasses are tested as a complete unit in order to comply with British and European Product Standards. These standards are not a legal requirement but are there to provide assurance of fitness for purpose. Tampering with the glasses to remove the original lenses and replacing them with prescription lenses would invalidate conformity with the standard and re-testing of the glasses once the lenses had been replaced may well be an expensive process.
The supplier should have explained this properly to the customer rather than saying it could not be done because of health and safety.